High performance resonant element

ABSTRACT

A method of forming a semiconductor device for processing a signal includes providing a circuit board including an input signal line, providing a high performance resonant element connected to the input signal line, and providing an output signal line connected to the high performance resonant element. The high performance resonant element includes a via.

The present application is a Divisional Application of U.S. patentapplication Ser. No. 11/450,583, filed on Jun. 12, 2006, the entirecontent of which is incorporated herein by reference.

This invention was made with Government support under Contract No.:H98230-04-C-0920 awarded by the National Security Agency. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a high performance resonantelement. In particular, the present invention relates to methods andsystems for processing a signal using a high performance resonant via.

2. Description of the Related Art

Signal lines and vias are both ubiquitous in circuit board and chippackage technology. A signal line is capable of transmitting a broadbandsignal with high fidelity, while a via is necessary to connect signallines that may reside on different levels within a laminated circuitboard or to bring a signal from a signal line embedded between layers ofa circuit board to the surface of the circuit board.

A via may be formed by drilling a hole completely through a circuitboard and then plating the interior of the hole with a conductivematerial. In this manner, a via may form a hollow, conductivebarrel-shape.

A via may be used to connect a signal line within a laminated stack ofpatterned layers within a circuit board to another signal line withinthe circuit board. FIG. 1 is a cross-sectional view of a circuit board100 having plane layers 102, a pair of dielectric layers 104, and asignal line (signal trace) 106 between the pair of dielectric layers104.

FIGS. 2A and 2 b provide cross-sectional views of two other circuitboards 200 and 202 to illustrate two different types of vias and theirrelationships to signal lines that may be distributed across multiplelevels within a circuit board.

FIG. 2A illustrates a through-type via 204. A through-type via 204receives a signal from a signal line 206 at one level of the circuitboard 200, carries that signal across substantially the entire length ofthe via 204, and provides the signal to another signal line 208 atanother level in the circuit board 200.

Electrically a through-type via is the most benign type of via. In otherwords, a through-type of via generally has the least amount ofdetrimental effect upon a signal in comparison with other types of vias.

FIG. 2B illustrates a stub type of via 210. The stub via 210 receives asignal from a signal line 212 at one level in the circuit board 202 andprovides that signal to another signal line 214 at substantially thesame level within the circuit board 202.

One characteristic of a stub type of via is that there is a long sectionof via (hence the “stub” moniker), which is not necessary forpropagating the signal. However, a stub via is generally unavoidablebecause of manufacturing cost constraints.

Stub vias are conventionally viewed as having very adverse effects uponsignal propagation. Therefore, circuit designers conventionally furtherprocess these signals to address the adverse effects that these viashave upon the signal being transmitted. If it is not feasible to do sothen, the signal impairments introduced by the stub vias may restrictthe frequency content of the signal that can be transmitted. This oftenleads to a reduction in the signal bandwidth that the connection cansupport.

In particular, as illustrated in FIG. 2B, when a signal enters a stubvia, the signal splits into two paths. One of those paths goes directlyfrom the input signal line 212 to the output signal line 214. However,the other path entails a portion of the signal traversing the fulllength of the via 210, reflecting from the other end of the via,traversing back along the full length of the via, and then splitting asecond time. One portion of that reflected signal returns to the inputsignal line 212 and the other portion of the reflected signal is carriedonto the output signal line 214.

The portion of the reflected signal that is carried onto the outputsignal line 214 represents a delayed and attenuated replica of theoriginal incident signal and will serve to contaminate subsequentsignals traveling down line 214. Such effects may be particularlydisadvantageous to a sinusoidal signal, and if the transit time, downand back from the end of the stub via is substantially equal to one-halfof the period of the sinusoidal signal, then phase cancellation of thesignal may completely attenuate the signal received from the inputsignal line 212.

Another type of via called is a “buried-type” of via (not shown) whichdoes not extend to one of the outer layers of a circuit board. However,while it is possible to construct a buried-type of via, these types ofvias require that holes be drilled in patterned layers before the layersare laminated together to form a circuit. This is a much more expensiveprocess than drilling the holes after laminating the entire circuitboard. Therefore, buried-type vias are not preferred.

Yet another type of via, called a “blind” via extends to one surface ofthe board and terminates on one of the internal layers of the circuitboard. A blind via can be used to circumvent a via stub effect as well.However, a blind via also suffers from higher fabrication costs, and dueto processing limitations, usually can only access layers close to oneof the circuit board surfaces.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, drawbacks, anddisadvantages of the conventional methods and structures, an exemplaryfeature of the present invention is to provide a method and system forprocessing a signal using a via.

In a first exemplary aspect of the present invention, a system forprocessing a signal includes an input signal line, a high performanceresonant element connected to the input signal line, and an outputsignal line connected to the high performance resonant element. The highperformance resonant element is at least one via.

The inventors discovered that the characteristics of vias may be viewedas an advantage, rather than as problems to be overcome as hasconventionally been done.

An exemplary embodiment of the present invention provides a via that mayact as a relatively high performance resonant element that may beadvantageously used for a large number of applications.

Conventionally, the characteristics of these vias may be selected suchthat losses are minimized. In stark contrast, in accordance with thepresent invention an exemplary embodiment takes advantage of thecharacteristics of a via by relying upon the via to process a signal.Since the incremental costs of vias in circuit boards are small, thereis little impact to the overall cost of the finished assembly. Thesignal processing afforded by these structures may be adequate to allowdedicated components to be entirely replaced with significant costsavings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary purposes, aspects and advantages willbe better understood from the following detailed description ofexemplary embodiments of the invention with reference to the drawings,in which:

FIG. 1 illustrates a cross-sectional view of a circuit board 100;

FIG. 2A is a cross-sectional view of a circuit board 200 with athrough-type of via 204;

FIG. 2B is a cross-sectional view of a circuit board 202 with a stub via210;

FIG. 3A is a model of one exemplary embodiment of the present invention;

FIG. 3B is a plot of a frequency response for the embodiment of FIG. 3A;

FIG. 4A is a model of another exemplary embodiment of the presentinvention;

FIG. 4B is a plot of a frequency response for the embodiment of FIG. 4A;

FIG. 5 is a plot of a frequency response for yet another exemplaryembodiment of the present invention;

FIG. 6 is a plot of three frequency responses for three additionalexemplary embodiments of the present invention;

FIG. 7 illustrates a coupler structure 700 in accordance with anexemplary embodiment of the present invention;

FIG. 8 illustrates a plan view of a circuit board in accordance with anexemplary embodiment of the present invention; and

FIG. 9 illustrates the frequency responses for the two circuit devicesin the circuit board of FIG. 8.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1-9, thereare shown exemplary embodiments of the methods and systems of thepresent invention.

Stub vias may be classified into two types: an open stub via and ashorted stub via. An open stub via has an end, which is distally locatedwith respect to associated signal lines and which is not connected toany other conductor. In contrast, a shorted stub via has an end, whichis distally located with respect to the associated signal lines, andwhich is connected to a ground.

Propagation of a signal within a via segment, is often modeled as apropagation down a transmission line, which, in general possesses adifferent impedance from “real” signal lines. FIG. 3A illustrates amodel 300 for a shorted-type via, and FIG. 3B illustrates a plot of afrequency response of the model of FIG. 3A.

The model 300 includes an input signal line 302, an output signal line304, and a shorted-type via 306. The via portion of the model includesan impedance element 308 in series with a resistor 310 and a capacitor312, which are parallel to each other and are connected to a ground 314.

While FIG. 3A illustrates a model 300 with specific elements, one ofordinary skill in the art understands that there are many degrees offreedom for forming a via, such as, for example, the drill diameter, thetype of conductive material, the length, and the like. Therefore, one ofordinary skill in the art understands that the elements which may beused to model the signal processing characteristics of a via may bevaried in accordance with the desired accuracy of the model and theresulting complexity of the model.

FIG. 3B is a plot of the frequency response of the model 300 of FIG. 3A.The plot of FIG. 3B illustrates the transmission of a signal of varyingfrequencies through the shorted-type of via 306. To model a shorted-typeof via, the resistance of the resistor 310 is set to a resistance ofzero ohms. Therefore, as illustrated in FIG. 3B, since the via 306 isshorted to ground 314, no direct current signal may pass through themodel 300.

FIG. 3B illustrates that, as the frequency of the input signal isincreased, because of the phase-delay between ground 314 and the inputsignal line 302, the ground 314 will not appear to be a ground at thejunction of the two signal lines 302 and 304. For the plot of FIG. 3B,at approximately five or six gigahertz 320, the via 306 passessubstantially all of the signal.

However, at a frequency of approximately 10 gigahertz 322, the via 306again acts as a short. Therefore, the frequency response of theshorted-type via 306 exhibits a periodicity alternating between a shortand an open at the junction of 302 and 304 because of the round triptransit time across the via.

In another exemplary embodiment of the present invention, thecharacteristics of the via 306 may be modeled with a resistor 310 thatis assigned a value other than zero. If this value is much greater thanthe characteristic impedance of the signal lines (strip lines) 302, 304and the via barrel 308, the model 300 more accurately reflects thecharacteristics of an open stub via 306.

In that instance, a direct current signal may pass directly through themodel 300 substantially un-attenuated. Indeed, the higher the resistanceof the resistor 310, a greater fraction of a low frequency signal willpass from the input signal line 302 to the output signal 304 withoutbeing affected by the via 306. However, for increasing frequencies, theopen stub configuration will eventually present a short to the junctionof striplines 302 and 304. This will occur first at the quarter-wavestub frequency when the phase due to the round trip delay traversing 308will add up to 180 degrees and lead to phase cancellation of theincoming signal. This will also occur at odd harmonics of thisfrequency.

Therefore, an open stub via generally exhibits characteristics, whichare complementary to a shorted stub via. For a direct current signal, anopen stub via generally allows the signal to pass un-attenuated, while ashorted stub via would attenuate substantially all of the direct currentsignal. Therefore, a open stub via acts like a low-pass filter while theshorted stub via acts like a high-pass filter.

Therefore, a single via has a frequency response that includes a singlenotch at one frequency that repeats across harmonics of that frequencyand which has a very high “Q” factor.

In another exemplary embodiment of the present invention, several viasmay be coupled together to provide an interacting ensemble of tightlycoupled high Q resonators.

Referring now to FIGS. 4A and 4B, another exemplary embodiment of thepresent invention is illustrated. FIG. 4A schematically illustrates acircuit 400 that has four shorted-type vias 402, 404, 406, and 408. Thecollective behavior of these four shorted-type vias 402, 404, 406, and408 provides a frequency response that is illustrated by the plot ofFIG. 4B. This exemplary embodiment illustrates characteristics, whichare desirable, for example, for use as a band-pass filter having apass-band 414. The broadening of a sharp reject notch into a stop band410 is due to the interaction of the resonances between the vias 402,404, 406, and 408.

Note that the frequency response of FIG. 4B has a periodicity with asecond passband 412 centered at 15 GHz. This second band 412 is usuallysignificantly attenuated due to losses in the structure, but may passsufficient energy that an additional roofing filter may be neededdepending on the application as understood by those of ordinary skill inthe art.

Comparing the frequency responses of FIGS. 4B and 3B, the rejection band410 of FIG. 4B is wider than the rejection band 302 of FIG. 3B. Also,the pass band 414 of FIG. 4B is flatter than the pass band 304 of FIG.3B.

FIG. 5 illustrates a plot of a frequency response for another exemplaryembodiment of the present invention that includes four stub vias (notshown). In comparison with FIG. 4B, the frequency response of FIG. 5exhibits substantially complementary characteristics. In other words,the frequency response of this exemplary embodiment exhibits low-passfilter characteristics.

These exemplary embodiments might not exhibit perfect band-passcharacteristics because each of these embodiments will exhibit imagecharacteristics. These band-pass filters may pass and/or reject allfrequencies, which are harmonically related. Therefore, one of ordinaryskill in the art understands that additional filtering may beadvantageous to process the images according to the specificapplications.

In another exemplary embodiment of the present invention, thecapacitance of a via may be controlled, by, for example, connecting oneend of a varactor having a voltage tunable capacitance to an open end ofthe via and the other end of the varactor to a ground. A varactor isadvantageous because it may be electronically tuned and, therefore, mayprovide an adaptive filter capacity. In this manner, by tuning thecapacitance, the frequencies at which the reject/pass bands occur may beadjusted. The response to such adjustments of a frequency response foran exemplary embodiment, which includes stub vias is illustrated by FIG.6.

Alternatively, the tunable capacitance may be implemented as lasertrimmable components or copper patterns on the circuit board itself, inwhich case, a trimming operation can be performed as a part of a testsequence.

FIG. 6 includes a first plot 600 of a frequency response for a pluralityof stub vias having no added capacitance, a second plot 602 of afrequency response for a plurality of stub vias each having acapacitance of 0.2 pF, and a third plot 604 of a frequency response fora plurality of stub vias each having a capacitance of 0.4 pF. As isclearly illustrated by FIG. 6, the pass/reject bands of each of thefrequency responses may be adjusted by adjusting the capacitance. Forexample, varactors added to the via stub terminus can provide a variablecapacitance by adjusting a direct current bias. In this manner, anexemplary embodiment of the present invention is capable of tuning theposition of pass/reject bands on a frequency spectrum if the fundamentaloperation for the circuit requires tuning or if it is desired to tunethe frequencies to improve yield.

A via acting as a band pass filter to reject cross-talk in accordancewith an exemplary embodiment of the present invention may beparticularly useful in radio frequency applications. For example, aradio frequency device may have a very powerful narrow band jammer witha transmitter transmitting at a specific frequency band. By providing avia in accordance with an exemplary embodiment of the present invention,which includes characteristics of a band-pass filter having a notch,which corresponds to the specific frequency band, then the via mayprovide significant rejection (attenuation) of that signal and mayprevent that signal from progressing into the signal lines.

Alternatively, in another exemplary embodiment of the present invention,a narrow band pass filter may be provided. A narrow band pass filter maybe quite useful for clock lines where it is desired to distribute aclock signal which only has a single frequency component and where it isdesired to reject any other noise which becomes coupled with the signal.This may prevent extraneous noise from coupling into the clock line,which would otherwise contaminate the clock signal and result inincreased phase noise.

Vias that are placed in close proximity to each other within a circuitboard may also transfer significant amounts of energy to each other. Anexemplary embodiment of the present invention may rely upon thiscoupling to form the basis of band limited couplers in which twotransmission lines, each with a periodic array of via stubs, are placedin close proximity as described below. In order to help confine energyto the coupler structure, it may be desirable to surround the structurewith a fence of grounded vias.

FIG. 7 illustrates an exemplary embodiment of the present invention thatmay be useful as a coupler structure 700. This exemplary embodiment ofthe present invention includes two arrays of vias 702 and 704 in closeproximity to each. Each array of vias 702 and 704 is connected to asignal line 706 and 708, respectively. In this exemplary embodiment 700,the energy between the lines 706 and 708 are coupled because, at a highenough frequency, the energy which traverses along each via of eacharray of vias 702 and 704, no longer stays within the via. Rather, theenergy received by one of the array of vias 702 or 704 may betransmitted to the other one of the array of vias 704 or 702,respectively. Above a certain frequency, each via acts like an antenna,which propagates energy into adjacent vias. In this manner, energy maybe transferred from one array of vias 702 or 704 to another array ofvias 704 or 702, respectively.

Coupler structures similar to the exemplary embodiment of FIG. 7 may beused in, for example, radio frequency (RF) applications.

FIG. 7 also illustrates that this exemplary embodiment 700 includes anarray of “guard” vias 710 which may surround the coupler via arrays 702and 704 and be connected to a ground (not shown) and, which, therefore,absorb energy and prevent energy from escaping from the coupler 700. Inthis manner, the guard vias 710 may protect other circuits (not shown)from having their signals corrupted by noise from the coupler 700. Inaddition, confining the energy to the vicinity of the coupled via arrayswill increase the coupling and selectivity of the coupler structure.This is due to the creation of standing wave patterns produced bycoherent scattering by the array of guard vias. These standing wavepatterns also present an opportunity to introduce additional frequencyselectivity by placing the coupler via arrays at appropriate nodes andanti-nodes within the standing wave patterns.

The via arrays comprising the coupler structure can be tuned using thetechniques presented in FIG. 6 to adaptively control the frequency overwhich coupling is significant as well as its strength. Such tuneabilitycan be used to advantage in many signal processing applications.

FIG. 8 is a plan view of a circuit board 800 that includes two moreexemplary embodiments of the present invention. One of the exemplaryembodiments of FIG. 8 includes shorted-type vias 802 connected to eachother by a signal line 804 and the other exemplary embodiment includesstub vias 806 connected to each other by a signal line 808. At the endof each of the signal lines 804 and 808, is a U-shaped launch structure,810 and 812, respectively.

FIG. 9 illustrates the frequency responses for each of the exemplaryembodiments incorporated into the circuit board of FIG. 8. As is clearlyillustrated, the frequency response 900 of the open stub via embodimentshows that at low-frequencies, an input signal is passed through thecircuit, while at around five to six gigahertz the signal issignificantly attenuated and, indeed, exhibits very good rejection ofthat band of frequencies. The frequency response 900 exhibits and imagepassband at about twelve gigahertz, but is attenuated by twelvedecibels. One of ordinary skill in the art would understand thatfrequencies above about twelve gigahertz may require further filteringdepending upon the particular application. Additional filtering may beneeded if the image passbands allow undesired spectral content of thesignal to leak through. If such spectral energy is not significant inthe signal, then such measures are not needed.

FIG. 9 further illustrates that the images (harmonics) of the frequencyresponse at the higher frequencies do not reach the original value ofsignal strength. This is the result of the loss (or transmission) ofenergy away from the vias at the higher frequencies as well as from thelosses in the transmission line.

The other frequency response 902 shown by FIG. 9 corresponds to theexemplary embodiment of FIG. 8 that has multiple shorted-type vias. Thisfrequency response 902 illustrates the characteristics of a band-passfilter from about five to ten gigahertz. Similarly, the frequencyresponse 902 exhibits excellent rejection at other frequencies andimages (harmonics) having a reduced output.

In addition to the characteristics of the vias which may be controlledas discussed above, one of ordinary skill in the art understands thatthe signal processing characteristics of a via may be adjusted anynumber of different ways while still practicing the present invention.For example, the Z0 (impedance), length (through back milling ordrilling), complex Z (pad and antipad geometries), and the like may beadjusted to tailor the response characteristics of a via in accordancewith the present invention. For example, the diameter of the via inconjunction with the diameter of the antipads will influence theimpedance of the via. Adjusting the overall length of the stub throughbackdrilling will affect the round trip delay in a direct fashionthereby providing a mechanism for scaling the frequencies of thepassbands and stopbands. Further fine tuning of the via characteristicscan be exercised with the addition of pads on unused signal layers andalso on the plane layers in the circuit board. In addition, the preciseplacement of GND vias located in close proximity can be varied to lendadditional fine tuning to the frequency response.

Further, the spacing between vias and the number of vias in a cascadedevice may also be adjusted to tailor the response characteristic of thedevice in accordance with the present invention.

An exemplary embodiment of the present invention may also utilize thebandpass characteristics of a via to reject cross-talk for any givenapplication. In such an application, if the characteristics of the viaare selected such that the rejection band is centered around a strongclock frequency, be it the fundamental or a harmonic, cross-talk fromthat clock will be attenuated by the via and attenuated as it propagatesalong a signal line. If this signal line is used to carry data, then theeye opening at the receive end will be improved and the effect of theclock cross-talk will be mitigated.

Another exemplary embodiment of the present invention may select thefeatures of a via such that the via serves as a narrow band pass filterwhich may be useful to distribute a clock signal. By selecting thecharacteristics of the via, the passband of the via may be tuned suchthat the passband is narrow and is centered at the fundamental clockfrequency. In this manner, the via will selectively filter out noisefrom the clock signal.

If the interfering signal is broadband, as is usually the case, most ofthe cross-talk will be attenuated in accordance with this exemplaryembodiment of the present invention.

Further, the frequency response of an array of vias may be adjustedbased upon variations in the terminations of the vias, by, for example,connecting the vias to varactors, adjusting the length of the via duringmanufacturing, and the like.

While the invention has been described in terms of several exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification.

Further, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

1. A method of forming a semiconductor device for processing a signal,said method comprising: providing a circuit board including an inputsignal line; providing a high performance resonant element connected tosaid input signal line; and providing an output signal line connected tosaid high performance resonant element, wherein said high performanceresonant element comprises a via.
 2. The method of claim 1, furthercomprising reducing a length of said via.
 3. The method of claim 2,wherein said reducing comprises one of backdrilling and milling of saidvia.
 4. The method of claim 2, wherein said reducing comprises adjustinga thickness of said circuit board.
 5. The method of claim 1, whereinsaid signal comprises a plurality of frequency components.
 6. The methodof claim 5, wherein said via processes said plurality of frequencycomponent to provide a desired output signal.
 7. The method of claim 6,wherein said via attenuates at least one of said plurality of frequencycomponents.
 8. The method of claim 7, wherein said at least one of saidplurality of frequencies comprises a noise frequency.
 9. The method ofclaim 7, wherein said via permits at least one other of said pluralityof frequency components to pass.